Not very. They just give a general guide. I have one which has green( no alcohol) Amber(alcohol present) and red(definitely too much to drive) lights on it.
I think the only use they have are for the day after you've had a drink. Like CAS said...just to see if alcohol is still present. Whether you're actually fit to drive is another matter...
Of course the readings you take from them may or may not be reliable.(Probably not)
However,the Police breathalyser is the one you have to worry about,as the readings from your one are not admissable in a court of law!
Just drink less,and keep well within the legal limit!
I personally favour the view that if you want to drive then you should have NO alcohol in your blood,a zero reading in other words.
My DH used to have one given by his work. If he was called into work unexpectedly he could use it to see if he was okay to go as the rule there was NO alcohol. All it meant though was if got called, he tested himself at home, got green and went in (eg on a saturday morning) if he got randommed at work where they use police quality ones, if he got amber then he was protected.